Mechanistic Studies of the Cu(OH)+‐Catalyzed Isomerization of Glucose into Fructose in Water

The isomerization of glucose to fructose is a crucial interim step in the processing of biomass to renewable fuels and chemicals. This study investigates the copper‐catalyzed glucose–fructose isomerization in water, focusing on insights into the roles of the dissolved copper species. Depending on the pH, the thermodynamic equilibrium shifted towards one or a few copper species, namely Cu2+, Cu(OH)+, and Cu(OH)2. According to thermodynamics, the highest concentration of Cu(OH)+ is at pH 5.3, at which the highest fructose yield of 16 % is achieved. The obtained fructose yields strongly correlate with the concentration of Cu(OH)+. A pH decrease of 2–3 units was observed during the reaction, resulting in the deactivation of the catalyst through hydrolysis in acidic media. Based on the results of the catalytic experiments, as well as spectroscopic and spectrometric studies, we propose Cu(OH)+ as an active Lewis‐acidic species following an intramolecular 1,2‐hydride shift. Brønsted‐pHobia! Catalytic experiments with different starting pH reveal the influence of the presence of Brønsted acid species on the activity of Lewis‐acidic Cu(OH)+ for glucose–fructose isomerization in an aqueous medium. A strong decline in activity is observed at low pH owing to the hydrolysis of Cu(OH)+ to inactive Cu2+, whereas a high pH leads to precipitation of inactive Cu(OH)2.